Journal of Experimental Botany, Vol 49, 1529-1537, Copyright © 1998 by Oxford University Press
A Gebler, M Schultze, S Schrempp and H Rennenberg
In the present study two experimental approaches were used to investigate
the influence of changes in the allocation of amino compounds in the phloem
of beech (Fagus sylvatica L.) seedlings on nitrate net
uptake by the roots. In a first set of experiments Gin or Asp were directly
fed into the phloem of the epicotyl via bark flaps. These compounds were
previously found to be allocated in the phloem of adult beech trees and
were shown to inhibit nitrate net uptake when supplied to beech roots.
Feeding of solutions containing 100 mM of Gin or Asp plus 10 mM EDTA into
the phloem resulted in a significant enrichment of the fine root tissue
with the amino compound fed as compared to the roots of control plants
supplied with amino acid-free EDTA solutions. Nitrate net uptake by the
roots decreased by 61% (Gin) and 79% (Asp) as compared to the controls. In
a second approach, shoots of young beech seedlings were exposed to
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ARTICLES
Interaction of phloem-translocated amino compounds with nitrate net uptake by the roots of beech (Fagus sylvatica) seedlings
Institut fur Forstbotanik und Baumphysiologie, Am Flughafen 17, D-79110 Freiburg, Germany; Corresponding author; Fax: +49 761 2038302; E-mail: here@sun2.ruf.uni-freiburg.de
g NH3 m-3. NH3
uptake by shoots, nitrate net uptake by roots, and the contents and
composition of total soluble non-protein nitrogen (TSNN) in leaves, phloem,
and fine roots were determined and were compared to results gained with
control plants exposed to charcoal-filtered air. NH3 fumigation of the
shoots of beech seedlings resulted in a 35% reduction of nitrate net uptake
by the roots as compared to controls. TSNN contents in leaves and phloem
exudate of NH3-fumigated plants increased by 56% and 37%, respectively.
This enrichment was mainly due to Arg and Glu in the leaves and Asp, Asn,
Glu, and Gin, but not to Arg, in phloem exudate. The TSNN content of the
fine roots was not changed by NH3 fumigation, but a significant increase in
the Gin content was observed. From these results it is concluded that
phloem transport of amino compounds, especially of Gin and Asp, from the
shoot to the roots mediates regulation of nitrate net uptake by the roots
of beech trees in order to adapt this process to the nitrogen demand of the
whole plant.Keywords: Amino compounds, phloem
transport, nitrate net uptake, NH3 fumigation, regulation, Gin, Asp, Arg.
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